Peavey Combo 300 - scratches with notes

[PRNDL]

I have a Peavey Combo 300 bass amp that scratches with low frequency bass notes. It's a solid state amp.

I'm wondering where to start and what to look for.
I began with testing the electrolytics, which seemed fine.

Low frequency notes from my function generator (on 100 Hz) did break up rather badly. Is that too low in frequency for a bass amp, or is that a sign of something?

(I'm more familiar with tube amps and effects)

Does anyone have a schematic?

Thanks!


Hmmm ... the speaker wire was cut and soldered, but they used some paper tape that was very wet.
Could it be this simple?

Also, it does seem to distort easily -- the comp light comes on when this happens.
Is this a compression indicator?

[Enzo]

What would you do if it were a tube amp? Isolate the problem. Is it the amp or the speaker? Is it the preamp or the power amp? Is it before or after the patch jacks? SCope the power supplies - are they up to voltage and free of excess ripple? Scope the output signal and the signal stage by stage. Is it clipping? Is it missing half the waveform? Just what? None of these things care if it is tube or solid state.

The powr amp card supplies the preamp with +/-15v. There is also a separate +/-15v zener regulated suply for the power amp itself. The main power amp runs on +/-50v. Aer all these OK?

ANy amp is a row of stages - tube or solid state - so trace the signal from the oputput pins of each stage through the amp.

Specifically I look for large DC offsets on the output pins of each op amp - pins 1 and 7. Most in/out jacks on PVs are protected by a pair of clamping diodes tied to the 15v rails. Look for shorted ones - and DC on a jack. It takes only a couple minutes to run down the board checking for DC on each op amp output, and then again checking every diode on the board for shorts.

The 400BH powr amp card schemo is already on www.ampix.org in the Enzo gallery, and PV will send you any schematic for the asking. COntact parts or customer service or repair there.

Guitars go below 100Hz, so I doubt a bass amp would have a problem with it.

I cannot imagine how the tape on a wire makes any difference.

[PRNDL]

I cannot imagine how the tape on a wire makes any difference.

The tape was over the soldered part of the wires and it was soaking wet. It might have been enough to cause minor weirdness.

I agree with you that it is unlikely to be the cause, and am testing the amp with my guitar and have asked the bassist to stop by and try it.

OTOH, the main problem was minor, which is always difficult if it's not easy to replicate. In this case I used a function generator, but the scratchy sounds may have been distortion with the overload light on. I had to read the manual to figure out how that works.

To me, it sounds a bit thick, which may be that it's a bass amp. Also, it just doesn't seem loud enough, although it may need more fiddling with the volume controls, as well as a place to turn it up without bothering neighbors. The bassist will be able to tell if it's right.

I'll check the opamps and diodes as you suggested.

Thanks!!!

[Gtr_tech]

A common problem with that chassis is the series inductor at the output (heavy wire with brown paper wrap). The wire can crack or completely break at the standoff from the PCB. Move it around and make sure it is secure or resolder it of broken. I never liked how they just left it hanging in the air like that....'specially in a bass combo amp....asking for trouble. They should've extended the wire and zip tied it down to the board.

[PRNDL]

Thanks ... I opened the amp to check and there wasn't a brown wire as described. The inductor was soldered to posts on the board.

Perhaps they redesigned the board and corrected that issue.

I really like that it has IC sockets, even though this one didn't have a bad opamp. It's a nice touch.

[Gtr_tech]

The whole coil should be layered on brown paper...although the magnet wire will likely be a reddish brown color.

[PRNDL]

Originally Posted by Gtr_tech

The whole coil should be layered on brown paper...although the magnet wire will likely be a reddish brown color.

Yes. It looked like a donut covered in brown paper with the two red wires soldered onto pins on the board.

[PRNDL]

I think the problem may be with low frequency bass notes. It sounds fine with guitar, but the owner assured me that it sounds bad with a bass and the last tech also had a function generator that didn't find the problem.

I do recall hearing bad sounds when I turned the frequency down on my generator to about 100 Hz.

My initial guess is bad electrolytics, beginning with the two power supply caps. Next up will be to look at the signal as it goes in and out of each of the opamps, in case one of them is losing frequency response.

I also wonder if it might have been the amps special clipping protection, which sounds horrible.

I've purchased a power resistor, which will make it much easier to test without listening to tones out of the speaker. Hopefully it will arrive soon since it's within the state.

------

Testing with my function generator failed to show any problems by ear. I'm going to have to let the owner show me exactly what's wrong using his bass guitar, and then I'll replicate the frequency and check it with the scope.

BTW - I wonder if it's the speaker. We had another Peavey with a 12" black widow that scratched with notes. Someone had taken the bell cover off, which could have let dust in to ruin it.

Is using another known good speaker the only way to test them?
I have several 8 ohm guitar speakers, but no 15" bass speakers.

[PRNDL]

The amp seems to have problems at about 80 Hz.

Using the function generator, it has a nice tone below about 75 and above about 85 Hz. At about 80 Hz, the amp has a scratchy distortion.

The FG isn't calibrated (neither is my scope), so the frequency might be near 60 Hz, which would indicate failing power supply electrolytics.

Or, is there something else going on?

BTW
The power resistors arrived today, so I can make a quiet test rig.

[Enzo]

Instead of guessing about the e-caps, scope the power rails. If they sag too much and the ripple goes nuts, then replace them. If teh rails look normal, then the trouble is elsewhere.

And scope the output, what does this distortion signal look like> what does it do to a sine wave?

I would never describe the clipping prevention circuit as sounding horrible. it is just a limiter that prevents the signal from advancing to clip in the power amp. Below the clip level, it is out of the way. CLipping sounds a lot worse than that limiting. Unless of course the limiter is what is wrong.

You can simply remove U2 from the power amp board - socketed I think - to disable and remove the limiter. A perfectly reasonable thing to try during tests.

Your generator may not be calibrated, but it is simple enough to compare the "80Hz" signal with 60Hz by ear.

Often when you find a certain note affects and amp more than others it is due to the sonic vibration in the cab. COnnect the chassis to the speaker with some long wires so the chassis is not in the cab with the speaker. Does that make a difference?

[PRNDL]

Thanks Enzo.

I followed your advice and everything in the amp checked out fine. The sine wave into the resistive load looked normal.

Another speaker (a 15W guitar speaker) didn't have the problem, which makes me believe that it's the speaker.

The bassist may bring over an external cab to try, but I'm pretty certain it's the speaker.

Has anyone had a speaker that's gone bad with only a narrow band of bass notes?
I thought it was unusual, but not unheard of.

As you may have guessed, I'm taking things slowly with solid state amps. I'm more familiar with tube amps and effects. It's pretty straightforward, which makes it easy to miss the forest through the trees.

[Enzo]

Certainly. A speaker is a mechanical system, and all those moving masses will have resonances. That it resonates at some particular freq and not others is not surprising.

[Steve Conner]

Just for reference, lowest note on a 4-string bass is 40Hz and on a 5-string it's 33Hz. I wouldn't expect a bass combo to reproduce either, but I'd expect it to make an effort without obvious rattling or clipping.

[Enzo]

40Hz, and at my age so does everything else.